According to the World Health Organization, respiratory diseases are the number one cause of world-wide mortality, with at least 20% of the world's population afflicted. Over 25 million Americans have chronic lung disease, making it the number one disabler of American workers (>$50 B/yr), and the number three cause of mortality.
Currently, most infections are treated with oral or injectable antiinfectives, even when the pathogen enters through the respiratory tract. Often the antiinfective has poor penetration into the lung, and may be dose-limited due to systemic side-effects. Many of these issues can be overcome by local delivery of the antiinfective to the lungs of patients via inhalation. For example, inhaled tobramycin (TOBI®, Chiron Corp, Emeryville, Calif.), is a nebulized form of tobramycin, that has been shown to have improved efficacy and reduced nephro- and oto-toxicity relative to injectable aminoglycosides. Unfortunately, rapid absorption of the drug necessitates that the drug product be administered twice daily over a period of ca., 20 min per administration. For pediatrics and young adults with cystic fibrosis this treatment regimen can be taxing, especially when one takes into account the fact that these patients are on multiple time-consuming therapies. Any savings in terms of treatment times would be welcomed, and would likely lead to improvements in patient compliance. Achieving improved compliance with other patient populations (e.g., chronic obstructive pulmonary disease (COPD), acute bronchial exacerbations of chronic bronchitis) will be critically dependent on the convenience and efficacy of the treatment. Hence, it is an object of the present invention to improve patient compliance by providing formulations with sustained activity in the lungs. Sustained release formulations of antiinfectives are achieved by encapsulating the antiinfective in a liposome. Improving pulmonary targeting with sustained release formulations would further improve the therapeutic index by increasing local concentrations of drug and reducing systemic exposure. Improvements in targeting are also expected to reduce dose requirements.
Achieving sustained release of drugs in the lung is a difficult task, given the multiple clearance mechanisms that act in concert to rapidly remove inhaled drugs from the lung. These clearance methods include: (a) rapid clearance from the conducting airways over a period of hours by the mucociliary escalator; (b) clearance of particulates from the deep lung by alveolar macrophages; (c) degradation of the therapeutic by pulmonary enzymes, and; (d) rapid absorption of small molecule drugs into the systemic circulation. Absorption of small molecule drugs has been shown to be nearly quantitative, with an absorption time for hydrophilic small molecules of about 1 hr, and an absorption time for lipophilic drugs of about 1 min.
For TOBI® the absorption half-life from the lung is on the order of 1.5 hr. High initial peak concentrations of drug can lead to adaptive resistance, while a substantial time with levels below or near the effective minimum inhibitory concentration (MIC), may lead to selection of resistant phenotypes. It is hypothesized that keeping the level of antiinfective above the MIC for an extended period of time (i.e., eliminating sub-therapeutic trough levels) with a pulmonary sustained release formulation may reduce the potential for development of resistant phenotypes. Hence, it is a further object of the present invention to maintain the ratio of the area under the lung concentration/time curve to the MIC at 24 hr (i.e., the AUIC), not only at an adequate sustained therapeutic level, but above a critical level, so as to reduce the potential for selection of resistant strains.
It is assumed that only the “free” (un-encapsulated) drug has bactericidal activity. One potential disadvantage of liposomal sustained release formulations is that the encapsulation of drug in the liposomal formulation decreases the concentration of free drug reaching the lung pathogens, drug which is needed to achieve efficient killing of bacteria immediately following administration. Hence, it is further an object of the present invention to provide a formulation that contains sufficient free drug, to be bactericidal immediately following administration.
The disclosures of the foregoing are incorporated herein by reference in their entirety.